Double negatives come up a lot in my field: the law. In California, for example, juries in criminal cases are instructed that "reasonable doubt" means something &ldquo;that leaves the minds of the jurors in that condition that they cannot say that they feel an abiding conviction . . . of the truth of the charge.&rdquo;

I realize this is an old question, but what neither the questioner nor any of the three previous answerers notice is that what is being discussed here is not the use of a double negative. A double negative is the use of two forms of negation in one clause, as in "I can't get no (satisfaction?). The sentence being discussed here uses two negatives, but in different clauses.

However, I agree with the questioner that it is an awkward sentence. It is not bad – or improper – English to repeat 'wasn't' in this case; the author is trying to use matching phrasal structures, which is a 'literary device'. What makes the sentence awkward is the intervening clause "in the sole agency contract," which interrupts the matched clauses.

I would have preferred: "In the sole agency contract, there wasn't a clause left that wasn't a source of conflict."